High-Temperature Wear Behavior of Overlay Hardfacing on 42CrMo Continuous Casting Rolls

Literature Overview

This 2021 paper by Cheng Jianguo and colleagues of Baowu Heavy Industry Co., Ltd., published in Materials Protection, investigates the high-temperature wear behavior of overlay hardfacing deposits on 42CrMo continuous casting rolls. The research was supported by the Anhui Provincial Key Research and Development Program (Project No. 1804a09020067), reflecting its significance in the steelmaking industry. Continuous casting rolls are subjected to extreme operating conditions: temperatures exceeding 800 °C at the contact surface, cyclic thermal loading, mechanical contact with molten steel, and continuous rotation. The overlay hardfacing layer is the primary defense against wear, and its performance under these conditions directly determines the roll's service life and the quality of the cast product.

Service Environment and Failure Mechanisms

The operating environment of continuous casting rolls can be characterized by several extreme factors:

Thermal cycling: The roll surface experiences repeated heating to 800–1000 °C and cooling to ambient temperature during each casting cycle, inducing thermal fatigue cracking.
Mechanical loading: Contact pressure between the roll and the solidifying steel shell reaches 20–50 MPa, with additional centrifugal forces in curved sections.
Chemical interaction: Molten steel and slag can infiltrate cracks in the overlay layer, accelerating degradation through oxidation and erosion.
Sliding wear: The relative sliding between the roll and the steel shell produces abrasive wear, with the severity depending on the hardness and microstructure of the overlay.

Failure Mode	Typical Temperature	Primary Mechanism	Visual Characteristic
Adhesive wear	600–800 °C	Transfer of material between surfaces	Smooth, shiny patches on roll surface
Abrasive wear	< 600 °C	Hard particles ploughing the surface	Parallel grooves and scratches
Thermal fatigue cracking	> 700 °C	Thermal stress cycling	Radial cracks from roll surface inward
Oxidative wear	> 800 °C	Oxide scale formation and spalling	Scale-like flakes on surface
Spalling	Variable	Combined thermal and mechanical fatigue	Large chunks of overlay detached

The authors identify that the dominant wear mechanism transitions from abrasive wear at lower temperatures to adhesive and oxidative wear at higher temperatures. This transition is critical for material selection and overlay design, as each mechanism requires different microstructural features for mitigation.

Overlay Composition and Microstructure

The study examines several overlay compositions, including high-chromium cast irons (e.g., 26Cr), martensitic stainless steels (e.g., 410, 440C), and nickel-based alloys. The microstructural evolution during high-temperature service is a key focus:

High-chromium irons: Contain primary carbides (M7C3, M23C6) dispersed in a ferrite or pearlite matrix. At elevated temperatures, the carbides can coarsen and the matrix can soften, reducing wear resistance. However, the high chromium content provides excellent oxidation resistance through the formation of a protective Cr2O3 scale.
Martensitic stainless steels: The martensitic structure provides high hardness (50–60 HRC), but the retained austenite fraction and carbide distribution significantly influence high-temperature stability. Temper embrittlement can occur during prolonged exposure at 400–600 °C.
Nickel-based alloys: Offer superior thermal stability and oxidation resistance, but at significantly higher cost. The microstructure typically consists of a γ-Ni matrix with M23C6 or Ni3B carbides.

The authors report that the overlay layer with optimal high-temperature wear resistance exhibits a fine, uniformly distributed carbide structure with a hardness of 55–62 HRC at room temperature and retains at least 80% of its hardness after exposure at 800 °C for 100 hours.

Process Considerations for Overlay Application

The overlay process used for continuous casting rolls typically involves submerged arc welding (SAW) or flux-cored arc welding (FCAW) with multiple passes. Key process parameters include:

Parameter	Typical Value	Rationale
Base metal preheat	200–300 °C	Reduce residual stress and prevent cracking in 42CrMo substrate
Interpass temperature	≤ 300 °C	Control grain growth and minimize softening of the base metal
Number of overlay passes	3–5	Achieve minimum thickness of 6–10 mm with controlled dilution
Post-weld heat treatment	600–650 °C for 2 h	Temper the overlay and relieve residual stresses
Surface finish after overlay	Ra ≤ 3.2 μm	Reduce initial wear rate during roll-up period

The dilution in the first overlay pass is typically 30–50% for 42CrMo substrates, which can significantly affect the overlay composition. The authors recommend using a "sacrificial" first pass with a composition closer to the base metal, followed by overlay passes with the target composition, to minimize the impact of dilution on the final overlay properties.

Engineering Practice and Case Study

In my experience with continuous casting roll refurbishment, the key challenge is not only achieving the desired overlay properties but also ensuring long-term bond strength between the overlay and the base metal. The thermal cycling experienced during service can induce interfacial cracking if the coefficient of thermal expansion mismatch between the overlay and the base metal is not properly managed. A practical approach is to select overlay materials with a thermal expansion coefficient close to that of 42CrMo (approximately 12 × 10⁻⁶/°C) to minimize thermal mismatch stresses.

The paper also highlights the importance of post-overlay machining. The overlay surface must be precisely machined to the required roll geometry, and the machining parameters must be optimized to avoid work hardening or residual stress introduction. A recommended approach is to use a two-step machining process: rough machining with deep cuts to remove the bulk of the overlay, followed by finish machining with light cuts to achieve the required surface finish and dimensional accuracy.

Key Reflections

This study provides valuable insights into the high-temperature wear mechanisms of overlay hardfacing on continuous casting rolls. The findings confirm that microstructural stability at elevated temperatures is as important as room-temperature hardness in determining service life. The practical recommendations for process parameters and post-weld treatment are directly applicable to industrial practice. However, the study could benefit from longer-duration wear testing at actual service temperatures, as laboratory tests may not fully capture the complex degradation mechanisms that occur during extended service.